1. Field of the Invention
The present invention relates to a process for completing, providing sand control and/or fracturing a subterranean well in a single trip, and more particularly, to the use of one or more perforating gun assemblies positioned within a screen assembly to permit perforation of a well and formation while fluid in the well bore is pressured to an predetermined condition, such as an overbalanced condition, and proppant is subsequently placed in the well without removal of the assemblies.
2. Description of Related Art
Production of unconsolidated materials, e.g. sand and other fines, from subterranean formations into wells is problematic. Left unabated, continued production of such unconsolidated materials can result in erosion of production equipment, well plugging, and/or reduced or complete loss of fluid production from a well. Thus, it is conventional practice to control the production of unconsolidated materials into many subterranean wells. Where the subterranean formation is composed of relatively hard, consolidated material and fracturing operations are performed so as to enhance fluid communication with the well, conventional practice is to control the flow of proppant that is utilized in the fracturing operations back into the well thereby ensuring that the fractures remained propped open.
In accordance with the most commonly practiced technique, “gravel packing”, a tubular liner is positioned in the well bore and a proppant gravel is placed in the annulus between the liner and the well bore. Gravel is commonly mixed with the fluid, such as a liquid or foam, to form a slurry which is pumped through a work string and a crossover tool into the annulus between the well bore and the liner. The slurry flows down the annulus to the bottom of the well bore or to a sump packer in the well bore. Some of the fluid of the slurry flows through the apertures in the liner into the open bottom end of a wash pipe situated within the liner and returns to the surface through the crossover tool and the annulus between the work string and the well casing. The bulk of the slurry fluid flows into the subterranean zone through perforations in the well bore. Gravel is thus deposited in the annulus and against the subterranean zone. The liner has slots or other apertures in its walls which are smaller in size than the gravel particles, thereby permitting formation fluids to flow through the slots while preventing entry of any unconsolidated materials. Gravel packing operations are typically performed at pressures below the formation fracture gradient, and the primary design considerations are placement of proppant inside perforation tunnels and in the annulus between the well bore and liner. The small apertures may be provided by a screen encircling the outer circumference of the liner tube, in which case the openings in the tube may be larger than the gravel particles. As a result of improved technology, gravel packs have become quite effective in excluding sand from oil and gas production. In addition to this function, the gravel also assists in supporting the walls of uncased wells and preventing caving of loose material against the liner. Despite the effectiveness of gravel packs once they are properly placed and operating, the procedure often results in undesirable completion skins or damage to the walls of the well bore which reduce the flow of formation fluids into a well.
In accordance with a relatively recent technique of completing well bores while practicing sand control termed “frac packing”, the unconsolidated formation is fractured and propping material is deposited in the fracture. Typically, a completion fluid of sufficient density for pressure control is first placed in a cased well, the cased well is perforated adjacent the subterranean zone or formation of interest. The perforating equipment is then removed from the well and a separate trip is required to place sand control equipment in the well adjacent the perforations. A fracturing fluid having proppant material incorporated therein is pumped, with the sand control equipment in place, at a sufficiently high pressure to propagate a fracture into the subterranean formation. The proppant materials within the fracturing fluid are deposited in the resulting fracture(s). While several variations of this process are practiced, the steps set forth above are employed to complete a given frac pack operation. However, significant costs are incurred with the material, equipment and time necessary to perform this series of operations.
The problems associated with conventional frac packing operations have spawned significant interest in reducing fluid costs, in developing simplified equipment and methods for minimizing the number of trips necessary to deploy equipment in the well and in eliminating the use of a rig at the surface of the earth. Methods and apparatus have been recently developed that allow perforating operations and screen placement to be performed in a single trip. U.S. Pat. No. 5,722,490 discloses a method of completing and hydraulically fracturing a well wherein a tubing conveyed perforating gun assembly is attached below a gravel pack screen. The perforating gun assembly is lowered to a depth opposite a productive zone and activated. The perforating gun assembly may be designed to be released from the tubing and fall to the bottom of the well after firing. The tubing string is then lowered to place the gravel pack screen opposite at least one of the perforations formed. Hydraulic fracturing operations are subsequently performed. However, this method still requires intervention with a rig to perform operations for positioning, perforating, setting of packer(s), etc. that are necessary to accomplish the method. Accordingly, a need still exists for a cost effective method for providing the stimulation benefits of a frac pack method together with sand control without necessarily requiring the use of a rig at the surface of the earth.
Methods have also recently been developed for exerting extreme pressures on a subterranean formation instantaneously with perforating the well casing so as to clean the perforation tunnels that are formed and to generate near-wellbore fractures to connect with existing natural fractures in the formation. U.S. Pat. No. 5,131,472 discloses such a method and provides for non-mechanical sand control by use of resin coated sand. However, a need exists for performing an overbalanced perforating operation while utilizing mechanical means and methods to provide for increased sand control, decreased time and costs and increased safety.